Let's roll up our sleeves and solve this. We have 998 and we're going to multiply it by 21, essentially adding 998 to itself 21 times.
Step 1: 0 + 998 = 998
Step 2: 998 + 998 = 1996
Step 3: 1996 + 998 = 2994
Step 4: 2994 + 998 = 3992
Step 5: 3992 + 998 = 4990
Step 6: 4990 + 998 = 5988
Step 7: 5988 + 998 = 6986
Step 8: 6986 + 998 = 7984
Step 9: 7984 + 998 = 8982
Step 10: 8982 + 998 = 9980
Step 11: 9980 + 998 = 10978
Step 12: 10978 + 998 = 11976
Step 13: 11976 + 998 = 12974
Step 14: 12974 + 998 = 13972
Step 15: 13972 + 998 = 14970
Step 16: 14970 + 998 = 15968
Step 17: 15968 + 998 = 16966
Step 18: 16966 + 998 = 17964
Step 19: 17964 + 998 = 18962
Step 20: 18962 + 998 = 19960
Step 21: 19960 + 998 = 20958

So, 998*21 = 20958